As a typical example of the force measuring instruments an electronic balance and a diaphragm-implemented measuring instrument are known in the art, wherein the diaphragm-implemented measuring instrument means a measuring instrument having a diaphragm on which an external force such as fluid pressure is applied. These measuring instruments include a balanceable scale beam which is displaced under a force exerting on a weighing pan and a diaphragm secured to the scale beam, and in response to the displacement, an electromagnetic force is developed by a transducer having a force coil placed in the static electromagnetic field so as to compensate the displacement. The strength of the electromagnetic force is detected. At this stage no equilibrium is yet reached between the force and the electromagnetic force. The detecting result is subjected to PID operation, and then fed back to the force coil so as to obtain an optimum strength of electromagnetic force for achieving the equilibrium. In this way the applied force is determined by measuring the feedback current. The "PID" is a known symbol standing for proportionality signal (P), an integral signal (I) and a differential signal (D).
However, the known system has the following problems and disadvantages:
First, the detecting result is represented in analog quantity, and the PID operation is performed in analog. The resulting signal is fed back to the force coil, and the current flowing through the force coil is converted into analog voltage signal through a resistance, and is then digitalized by an A/D (analog-to-digital) converter. The digitalized signal is supplied to a microcomputer, and the resulting measuring value is arithmetically operated like averaging, and displayed as a mean value.
In such an analog servomechanism it is required to employ an A/D converter having a relatively large number of bits so as to achieve high precision without sacrificing the dynamic range. As a whole the system becomes expensive because of the unavoidable provision of an A/D converter having a large number of bits and a high responsibility.
Second, the resistance through which the feedback current flowing through the force coil is converted into analog voltage signal generates heat. Owing to the heat the resistance values are varies, thereby resulting in an unstable span. The force coil also generates heat which is likely to raise the temperatures of the permanent magnet in the coil. Thus the magnetostatic field becomes unstable. Unstable magnetostatic field causes drifts in the measuring values.
Third, under the known analog servomechanism the achievement of stable control requires a lower proportional gain of the system but in order to enhance resolution a higher proportional gain is required. The two requirements for stability and resolution are contradictory with each other, and cannot be satisfied at the same time.
In order to overcome these disadvantages, there is a proposal for digitalizing the whole process of PID operation but a high-speed A/D converter having a large number of bits occupies a large space and is costly.